The present invention relates to relatively strong aluminum alloy products suitable for important applications such as airplane fuselage panels or parts and other applications and to improved methods for making such.
Heat treatable aluminum alloys are employed in many applications where high strength and low weight are desired. The 7XXX series of aluminum alloys (the Aluminum Association designates series or families of aluminum alloys by numbers as is well known) is very strong having typical yield strength (Y.S.) levels of 70 or 80 ksi or more. The term “ksi” refers to thousands of pounds per square inch; 80 ksi means 80000 pounds per square inch (psi). The 6XXX series of heat treatment aluminum alloys is not as strong as the 7XXX alloys but still has very good strength-to-weight ratio, quite good toughness and corrosion resistance, together with good weldability for many of the 6XXX alloys, in that 6XXX alloys after welding have good retention of mechanical properties, for instance, a higher percent retention in the weld zone than commonly used 2XXX or 7XXX alloys. Heat treatable alloys are solution heat treated at relatively high temperatures, quenched such as by water immersion or sprays and then artificially aged to develop their strength, as is well known. The products can be sold after quench and before artificial aging in a T4 type temper (solution heat treated, quenched and allowed to reach a stable naturally aged property level). The T4 type condition allows more ease of bending and shaping than the much stronger artificially (heat) aged T6 temper. The 6XXX series of alloys contain magnesium (Mg) and silicon (Si) as their main alloying ingredients, often also including lesser amounts of elements such as one or more of copper (Cu), manganese (Mn), chromium (Cr) or other elements. Alloy 6061 is commonly used for sheet and plate and forgings and 6063 is an old extrusion alloy in the 6XXX family. More recent alloys are 6009 and 6010 and are described in U.S. Pat. No. 4,082,578 to Evancho, and still more recent is alloy 6013 described in U.S. Pat. No. 4,589,932 to Park. The entire contents of both U.S. Pat. Nos. 4,082,578 and 4,589,932 are incorporated herein by reference. Alloy 6013 has been used in automotive and aerospace applications as well as others. It is recognized in the art as providing good strength, toughness, workability, corrosion resistance and good weldability so as to make it desirable for many uses. According to Aluminum Association limits, alloy 6013 contains aluminum and 0.6 to 1% Si; 0.8 to 1.2% Mg; 0.6 to 1.1% Cu; 0.2 to 0.8% Mn; 0.5% max. Fe; 0.1% max. Cr; 0.25% max. Zn; 0.1% max. Ti; not more than 0.05% each of other elements (0.15% total others), all percentages for aluminum alloy compositions referred to herein being by weight unless otherwise indicated. Alloy 6013 is typically produced by homogenizing at a very high temperature such as 1040° F. or so followed by hot rolling and, for thinner metal gauges, cold rolling, then solution heat treating at a high temperature such as 1040° F. or so, quenching and artificial aging.
Alloy 6013 is being thought about for use as large sheet or plate panels in very large commercial jet aircraft as fuselage panels, especially fuselage belly panels (belly panels are on the fuselage underside as is known), and possibly even larger fuselage portions such as most or even all of the fuselage. However, this potential use may be impended by a condition in 6013 sheet and plate products which appear as microscopic features under 500× magnification that look similar to pores but are not voids (pores are voids.). These features can also be found in other 6XXX alloys. These features are typically about 1 or 2 microns to about 5 or more (most being 2 to 5 μcm) microns (μm) in size referring to their major axis and can be detected by scanning electron microscopy (SEM) where they appear as microscopic “features” or pockets of reduced density in that they cause less reflection or backscattering of electrons than the surrounding metal which appears as normal density. Thus, the features might look like pores or voids at first but on more refined analysis appear as reduced or altered density features, that is, relatively solid but less dense than surrounding metal. Under SEM, the features appear as dark spots to suggest less density or at least less reflection of electrons in comparison to surrounding metal which reflects more electrons. In referring to reduced density features herein, such refers to appearance, under SEM examination preferably at an accelerating voltage of about 15 kilo-electron volts (keV or kV for short in SEM nomenclature) where the features are readily seen. (At 5 keV, the features are more difficult to see). The magnifications employed can vary from 500× to 10,000× although 500×is quite useful. Backscattered electron imaging is used rather than secondary electron imaging so as to provide higher contrast between the features and surrounding metal. These SEM techniques are all well known in the SEM art. Under SEM examination using backscattered electron imaging, a higher density site (such as one having elements of high atomic weight) reflects more electrons (looks lighter) than a lower density site, such as the reduced density features here described, which appear as darker spots. Magnesium silicide particles (Mg2Si) also can appear as dark spots under SEM because magnesium's atomic weight is lower than aluminum's but can be distinguished from the aforesaid reduced density sites by examining the X-rays emitted from the sample in the SEM using standard energy dispersive X-ray spectroscopy methods which are well known in the art. The reduced density features' composition differs quite substantially from Mg2Si in X-ray spectroscopy and is much more like the surrounding material composition albeit at lower density. In commercially produced 6013-T6, these features typically can number from around 50,000 or so to over 125,000 features or bodies in a square inch under 500× magnification in a metallographicly polished sample suitable for SEM. The sample can be taken at or near the mid-thickness plane but such is not necessary.
It is believed that these features apparently might act as weak spots during propagation of a crack such as in a fracture toughness test or more likely in a fatigue crack growth rate test or otherwise act adversely and it is considered very desirable to eliminate or reduce these features or defects. Thus, while 6013 type alloy sheet and plate are good products, they could be significantly improved by eliminating these features and thereby improving properties, especially by reducing fatigue crack growth rate.